A vitreous silica crucible is used for manufacturing a silicon single crystal. In the Czochralski method (the CZ method), polysilicon is charged into the vitreous silica crucible, and heated and melted to produce silicon melt. Thereafter, a seed crystal is dipped into the silicon melt, and the seed crystal is gradually pulled while rotating the crucible and the seed crystal in the opposite direction to grow a single crystal. In order to manufacture a high-purity silicon single crystal for a semiconductor device, it is required that a silicon single crystal is not contaminated by dissolution of impurities contained in the vitreous silica crucible, and it is also required that the vitreous silica crucible has sufficient heat capacity so as to easily carry out temperature control of the silicon melt. Therefore, a vitreous silica crucible having an opaque outer layer containing numerous microbubbles and a transparent inner layer whose bubble content rate is 0.1% or less (the average diameter of bubbles is 100 μm or less) is preferably used (See Patent Document 1). Furthermore, a vitreous silica crucible having two-layer structure is also used (See Patent Document 2). In such a crucible, the outer layer is formed of natural silica to enhance the high temperature structure of the crucible, and the inner layer which contacts the silicon melt is formed of synthetic silica to prevent mix-in of impurities.
Nowadays, along with enlargement of a silicon wafer, a vitreous silica crucible having an opening diameter of 700 mm or more is used. This leads to the increase of the amount of melting, long-time pulling over 100 hours or more, the increase of distance from the heater provided in the crucible to a silicon crystal, and the resultant increase of heating. These factors increases heat load to the vitreous silica crucible, and results in sidewall lowering where the lower portion of the vitreous silica crucible deforms by the crucible's own weight during pulling. In order to prevent the sidewall lowering, there is known a vitreous silica crucible whose inner surface or outer surface is strengthened by crystallization. For example, in Patent Document 3, a crystallization promoter is applied onto the crucible outer surface, and the crucible is strengthened by crystallization of the crucible during pulling. Furthermore, in Patent Document 4, oxyhydrogen flame is thrown to the crucible outer surface to form a crystallized vitreous silica layer on the crucible outer surface. Furthermore, in Patent Document 5, the entire inner surface of the vitreous silica crucible is polished by sandblast or the like, and the polished surface is subjected to heat treatment by oxyhydrogen flame for smoothing.